Discussion:
The results of our study suggest that TMPG correlates poorly with the severity of MS in patients with DMS. Moreover, DMSI may be a viable alternative for the evaluation of DMS. The potential inadequacy of mean TMPG reported here agrees with previous guidelines.2,17 However, our results note a significant discordance between TMPG and MVA in DMS patients. Differences in MV morphology and net atrioventricular compliance between DMS and RMS may, in part, explain this observation. The flat plate geometry caused by MAC, compared to the dome-shape of RMS, increases the contraction coefficient.14 An increased contraction coefficient causes a lower pressure drop across the valve, accounting for the low transvalvular gradient.10,11,14 Also, risk factors for an increased net atrioventricular compliance are much more common in the patients with DMS than RMS.11,20–23 An increased net atrioventricular compliance accelerates the rate of transvalvular pressure decay, hence lowering the mean TMPG.20,24 A recent study suggested the presence of diastolic dysfunction as a potential cause of low TMPG in patients with very severe RMS.25 The above factors may be responsible for lower mean TMPG seen in DMS patients in our study. The use of mean TMPG as an index for MS severity carries a 1B recommendation, but it may not accurately reflect MS severity in the setting of DMS.15–17.
The patients in our RMS and DMS groups were similar to each other regarding some of the hemodynamic (i.e., pulmonary artery systolic pressure, SV index) and structural (i.e., left atrial volume index) parameters. Despite this similarity between groups, DMS was associated with a larger MVA and lower mean TMPG compared to RMS. Consistent with our findings, Pressman et al. recently reported in a retrospective cohort study that DMS is associated with a larger MVA relative to mean TMPG when compared to RMS.26
The DMSI proposed here denotes an increased velocity across the stenotic MV relative of the LVOT velocity. Similar to the continuity equation, the DMSI relies on the rule of conservation of mass. The strong correlation between MVACEQ and the DMSI relies on the fact that the DMSI is a large component of the MVACEQformula. Despite these similarities, this DMSI has several theoretical advantages. It avoids the variability of the LVOT cross-sectional area and reduces error risk driven by LVOT diameter measurement and assumptions of LVOT shape. Thus, the index would be most useful in cases with suboptimal visualization of the LVOT.
The DMSI is relatively flow independent in the absence of significant valvular regurgitation. Therefore it would not be affected by the heart rate or flow state. The concept of the velocity ratio was previously described in the assessment of prosthetic aortic and mitral valve stenoses.27–29 The former utilized the ratio of LVOT VTI to prosthetic valve VTI with < 0.30 denoting severe prosthetic aortic valve stenosis, and the latter utilized ratio of mitral prosthesis VTI to LVOT VTI denoting > 2.5 as a significant valve dysfunction.29 Furthermore, the dimensionless index for the native aortic valve is calculated as a ratio of LVOT VTI to aortic valve VTI. Moreover, an index of < 0.25 is consistent with severe aortic valve stenosis.
A recent study by Cho et al. including RMS patients with very severe MS (MVA ≤ 1 cm2) undergoing MV replacement showed that patients with low mean TMPG (≤ 10 mmHg) were more likely to be older, and females, and have diabetes or AF compared to those with high mean TMPG (≥10 mmHg).25 The authors noted that this might be related to higher atrial compliance and lower atrial pressure seen in RMS patients. Interestingly, LV SV was higher in the low mean TMPG group in the study mentioned above. The frequency of low-flow state (SV index <35 ml/m2) was similar in our DMS and RMS groups. Therefore, between-group differences (mean TMPG and MVACEQ) observed in our study is unlikely to originate from differences in flow-states. We did not perform further subgroup analyses for patients with low-flow states because of the small sample size. Cho et al. reported an association between AF and low mean TMPG in patients with RMS.25 Absence of such a relationship in our study might be due to the small number of patients with AF in our DMS group (14%).
Our study shows that mean TMPG does not correlate well with MVACEQ and that a practical tool such as DMSI proposed in this study may serve as a simile echocardiographic parameter in the identification of patients with significant DMS.
Our survival analysis indicated that patients with DMS have a poor prognosis, and more than 50% of the patients with DMS died over an average of 13 months. The small sample size was a limitation for the detailed evaluation of predictors of mortality among patients with DMS. According to the cox-regression model, the left atrial volume index and ESRD were the only two independent predictors of mortality among DMS patients. A non-significant trend towards increased mortality risk was observed with MVACEQ ≤ 1.0 cm2 and DMSI ≤ 0.35. Other echocardiographic parameters such as MVACEQ ≤ 1.5 cm2, mean TMPG > 7 mmHg, or SV index ≤ 0.35 did not predict mortality risk.
Medical management of DMS includes heart rate control, and diuretic therapy that may cause provide some symptomatic relief.2 Surgical treatment for severe DMS is significantly challenging due to older age of these patients with multiple comorbidities.2 Emerging transcatheter approach for mitral valve replacement is feasible, but further advancement is needed to improve outcomes.30